3rd Generation Partnership Project (3GPP) based on Wideband Code Division Multiple Access (WCDMA) radio access technology has been widely developed worldwide. High Speed Downlink Packet Access (HSDPA), which may be defined as the first evolution of WCDMA, provides radio access technology having high competitiveness in the mid-term future to 3GPP. As a system for providing high competitiveness in the mid-term future, there is an Evolved-Universal Mobile Telecommunications System (E-UMTS).
FIG. 1 shows a network architecture of the E-UMTS. The E-UMTS is an evolved form of a WCDMA UMTS, and the standardization thereof is ongoing in the 3GPP. The E-UMTS is also called a Long Term Evolution (LTE) system. For the detailed contents of the technical specifications of the UMTS and the E-UMTS reference may be made to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”, respectively.
As shown in FIG. 1, the E-UMTS may include a User Equipment (UE), a base station (hereinafter, referred to as an “eNode B” or “eNB”), and an Access Gateway (AG) positioned at the end of the network (Universal Terrestrial Radio Access Network: E-UTRAN) and connected to an external network. Generally, the eNode B may simultaneously transmit multiple data streams, for broadcast services, multicast services and/or unicast services. One or more cells may exist in one eNode B. A plurality of eNode Bs may be connected by an interface for transmitting the user traffic or control traffic. A Core Network (CN) may include the AG and a network node for the user registration of the UE. An interface for distinguishing between the E-UTRAN and the CN may be used. The AG manages the mobility of the UE in the unit of Tracking Areas (TAs). The TA is composed of a plurality of cells. When the UE moves from a specific TA to another TA, the UE informs the AG that the TA of the UE is changed.